In a VLSI chip, both the core and the I/O blocks play an important role. The I/O blocks are arranged in the periphery in a ring-like fashion. To handle various signals like the power signals (high-voltage and low-voltage supplies and grounds), ESD signals, slew control signals, there are various rails passing above all the I/O blocks. However, these I/O blocks are spaced apart by a certain distance, the minimum of which is known as the pitch of the technology used. To ensure the continuity of these rails, some filler cells known as the IO-FILLERS are generally used. These filler cells contain nothing more than metal rails and ensure the continuity of rails in a non-stop ring-like fashion. The rails are generally in the top metal layers. However, the area occupied by these filler cells is not used for the fabrication of any transistor. Thus, the area of the metal rails is underutilized.
A concept of a voltage regulator is such that it contains a driver MOS (also called a pass transistor), whose size depends upon the load-current capability of the voltage regulator and is generally huge to provide current to the entire chip. This MOS needs an input supply VIN and generates an output supply VOUT, controlled by a voltage VCONT generated by a feedback circuit and an error amplifier as illustrated in FIG. 1. The output supply generated needs to be distributed in the entire chip. It is often not feasible to route the output supply to each and every corner of the chip, thus it is preferable to make use of the I/O ring in the periphery of the chip. This I/O ring will automatically route the supply VOUT around the periphery of chip. This is implemented by placing this driver MOS on the periphery of the chip so that it takes the input supply from one rail of the I/O ring and drives the output supply on another rail of the I/O ring, with the controlling voltage VCONT on a third rail.
The rails for VIN, the higher input supply, and VOUT, the lower output supply, are always present in an I/O ring with their corresponding grounds. In addition to these rails, there are certain dedicated rails in an I/O ring to take an external reference signal round the chip. One of these rails can be used to take the VCONT signal round the periphery of the chip to connect the gates of all the pass transistors together. In this way all the three nodes connected to the pass transistor are taken round the I/O ring with great ease.
Conventional voltage regulators have a bypass mode, where VIN is to be bypassed to the output node VOUT by pulling down the VCONT node to ground, and VIN applied is at the level of the VOUT itself. For example, in the bypass mode of a 5V to 1.2V voltage regulator, the voltage VIN, which is otherwise 5V, itself becomes 1.2V, and this voltage is then transferred to the VOUT node via the resistance drop of a switched-on PMOS. So the MOS sizes typically need to be huge to have a low on resistance.
A conventional technique employed for a VLSI chip containing a voltage regulator is illustrated in FIG. 2. Also shown in the figure is the lumped pass transistor, the output transistor of the voltage regulator, along with a number of pads to satisfy electromigration rules. This arrangement may suffer from severe drawbacks such as electromigration problems due to poor power distribution, high IR drops, and difficulty in routing to the core.
Another conventional structure shows distributing the pads and fractions of the pass transistor over the periphery of the chip. If the pass transistor has four such pads, then the transistor can be split into four parts with each part occupying a side of the chip along with a pad. The scheme is shown in FIG. 3. This provides better power distribution and lower IR drops, but a potential problem in this structure is that each such I/O can occupy more area than a standard I/O in order to accommodate the huge-sized pass transistor. The problem may get worse in the bypass mode of the voltage regulators, where much larger sizes of the pass transistor are typically needed as there is no regulation in the bypass mode, and the on-resistance of the pass transistor typically needs to be reduced significantly. Thus, this structure has drawbacks in pad-limited designs.
U.S. Pat. No. 6,594,809 is a prior-art patent pertaining to area utilization within the core of a chip. It relates to low-leakage diode insertion for integrated circuits, particularly to inserting diodes in filler cells in the core of the integrated circuits. A drawback of this patent is that it does not provide a solution for utilization of area on the periphery of chip.